There are known emitters where the infrared radiation emanates from a surface which is directly or indirectly heated by an electrical current passing through a conducting layer in or below the surface. The elevated temperature leads to increased emission of radiation, with the intensity and spatial as well as spectral distribution of the emitted radiation depending on the element temperature, as well as on its emissivity and surface topography.
For a greybody with emissivity .epsilon., the emitted power per unit area, within a wavelength interval d.lambda. at wavelength .lambda. is given by: ##EQU1## where T is the temperature, h is Planck's constant, k is Boltzmann's constant and c is the velocity of light.
In spectroscopic analysis, parts of the emitted power within restricted wavelength regions are selected by means of optical filters, and the radiation source is sought optimized by having a high emissivity .epsilon. and high temperature T.
In order to achieve compact, efficient and low-cost device solutions, it is desirable to switch or modulate the emitted radiation by rapidly varying the temperature T of the emitter, rather than by mechanical motion of shutters, filters etc. Electrically switched infrared emitters are also of great interest in many non-spectroscopic applications such as thermal printers, etc.
Two well-known types of pulsed thermal emitters in current use are:
1) Filaments in bulb-like enclosures. The filament is driven by a pulsating current, causing heating/cooling and associated variations in emitted infrared radiation. These sources are used in infrared sensors for gas monitoring, etc, and are relatively cheap. Unfortunately, their operative life is short, requiring frequent replacement. Also, they are bulky and consume much electrical power compared to the useful infrared radiation emitted. PA1 2) Electrically conducting, planar surfaces that are heated by a pulsating electrical current. The surface may be a flat, insulating substrate which is coated by a conducting film or layer, or it may be a thin, freely suspended membrane of a material which is itself electrically conducting. PA1 The former type of emitter is described in Norwegian patent No. 149,679 and U.S. Pat. No. 3,961,155. Drawbacks of these types of emitters include: Poor efficiency (i.e. much electrical power needed compared to the useful radiation emitted), which is in large part due to heat conduction through the relatively thick (typically 0.5 mm) substrate. Also, mechanical mounting and bonding of electrical connections is critical and labor intensive. PA1 The latter type has been implemented in commercial gas sensors, in the form of a silicone membrane which is etched thin and doped to high conductivity in a central region where infrared emission takes place. These sources have proven very robust and long-lived, and are generally more efficient than the emitters referred above, although the heat loss through the membrane to the mounting fixtures is still quite high. This is due to the thicker portions of the membrane, which must have a certain mechanical strength to permit an acceptable yield during manufacturing operations, as well as robustness in practical use. A serious drawback of these emitters is their high manufacturing cost. PA1 a) Radiation into the space in front and back of the membrane. PA1 b) Conduction through membrane mounts and electrical connectors. PA1 c) Thermal diffusion through gas surrounding the membrane. PA1 d) Convection in gas surrounding membrane. PA1 Inducing a rapid temperature rise during the application of electrical power to the membrane. This in turn is facilitated by high power dissipation and low heat capacity in the membrane. PA1 Achieving rapid cooling of the membrane during the &lt;&lt;power off&gt;&gt; period. This is achieved by a low heat capacity in the membrane, coupled with high heat loss from the membrane. A high heat loss from the membrane by mechanisms other than a) above, leads too poor efficiency, however.
The ideal thermal emitter should convert 100% of the dissipated electrical energy into infrared radiation at the desired wavelengths. Membrane emitters described above are a far cry from this; typically the ratio between total radiated power and supplied electrical power is 10% or less. The pulsating part of the radiated power is a fraction of the total emitted power, and the infrared emission within specific spectral bands is a fraction of this.
As shown schematically in FIG. 1, heat is generated in the thin membrane and can take several different paths:
Radiation is the heat loss mechanism which is sought maximized at the expense of the following:
Each of these can be quantified, subject to defined conditions such as materials, dimensions and operating temperature. Consider, e.g. the membrane shown in FIG. 2, which is freely suspended at opposite ends and surrounded by parallel surfaces representing the floor and the output window in a can filled with air: The different heat loss contributions a) to d) are shown qualitatively in the graph.
As is evident from FIG. 2, and by simple intuitive reasoning, the membrane emitter should have certain basic features:
First, in order to emit much infrared radiation, the membrane temperature and emissivity should both be high, cf. Equation 1). More specifically, the temperature should reach a high peak and a low trough value during each temperature cycle (cf. FIG. 3), to maximize the difference in emitted radiation between the &lt;&lt;power on&gt;&gt; and &lt;&lt;power off&gt;&gt; states. High temperature contrast is obtained by:
Summing up, the heat capacity of the membrane should be as low as possible, implying in practice that the membrane should be as thin as is compatible with mechanical strength requirements in the given application (vibrations etc). Furthermore, the surface facing the direction in which the radiation is to be emitted should have a high emissivity at the wavelengths of interest. Total heat transport from the membrane should be minimized, but must be sufficiently high to yield a sufficiently rapid cooling transient during the &lt;&lt;power off&gt;&gt; period. In the limit of an extremely thin membrane with near-zero heat capacity, heat loss by radiation is sufficient to cause a rapid cooling transient, and all other heat loss mechanisms b) to d) should be brought as close to zero as possible.
One notes that a thin membrane is all-critical, since this reduces the need for heat loss by other mechanisms than a), and leads to an efficient device requiring little electrical power. Heat transport along the membrane and to mounting posts/electrical leads is also minimized by using a thin membrane.
Membrane sources with their shortcomings as outlined above represent a compromise between the ideal technical requirements outlined above and the need for physical robustness and manufacturability at acceptable costs.